How to bend nurbs surfaces
I reckon you are new to 3d modeling , hence the question seems strange since you started out with a box. Nurbs modeling is a completely different beast. Let me try to help you out.

First of all, just like ibalazscs says nurbs are seldom used these days, its has proven itself to be quite a clumsy tool in regards to modeling. We deploy something else call "box modeling". Basically starting from a box or a simple plain we can create an estimate of an object surface and later add a modifier "Subdivision Surface"

This is what I mean ... on the left is a model that started out as a plane. I extruded the edges and created a draft form. After which I add a modifier to the model (Copied and selected highlighted in orange)...



After that there is a further step you may need to do to complete the look of a nice smooth curve surface.

The thing about curved surface in a 3d world is that you need more than one face to create the illusion of a curved surface. Yes illusion! All the surface you see in a 3d software are planes... using smooth surface shader those straight and flat planes hides their true edges, making them look like a smooth surface. Without smooth shader the model would look like this...



And here is the exact same model with the same number in vertex and face count, smooth shading applied...



Lastly, here are the steps to apply smooth shader...  


Select the model.  
Hit Tab to enter edit mode.  
Hit W to display special drop down menu.  
Select "Shade Smooth" from list.